Multi-layer microwave resonator

ABSTRACT

A multi-layer microwave resonator ( 10 ) comprises a cavity ( 12 ) having an inner surface formed from an electrically conductive material. Pieces ( 16   a   -16   e ) of dielectric materials stacked on top of each other form a conriguous body ( 14 ) that is provided in the cavity ( 12 ). The dielectric materials of the pieces ( 16   a   -16   e ) are chosen such that the dielectric constant of the pieces ( 16   a   -16   e ) alternate between a relatively high dielectric constant and a relatively low dielectric constant.

This application is a 371 of International Application No.PCT/AU00/01579, filed on Dec. 21, 2000, which designated the UnitedStates and which is published in English.

FIELD OF THE INVENTION

This invention relates to a multi-layer microwave resonator. In thisrespect, the term ‘microwave’ is used in this specification to denotethe range of frequencies that the invention may be useful, and includesmicrowave frequencies, millimetre wave frequencies and quasi-opticalfrequencies, in the frequency range of 1 GHz to 100 GHz.

BACKGROUND ART

Modern radar and telecommunications systems require high frequencysignal sources and signal processing systems with stringent performancerequirements and extremely good spectral purity. Thus, there is a needfor signal processing systems and signal sources with ever increasingspectral purity, stability and power-handling requirements.

Resonators, by their nature, provide discrimination of wanted signalsfrom unwanted signals. The purity and stability of the signals producedis directly linked to the resonator used as the frequency determiningdevice and is dependant upon its Q-factor, power handling ability andits immunity to vibrational and temperature related effects.

It is known that a piece of dielectric material has self-resonant modesin the electromagnetic spectrum that are determined by its dielectricconstant and physical dimensions. The spectral properties of a givenmode in a piece of dielectric material are determined by the intrinsicproperties of the dielectric material, its geometric shape, theradiation pattern of the mode and properties and dimensions of thematerials surrounding or near the dielectric material.

Prior art resonators have traditionally relied on metallic cavitiescontaining no dielectric material, or on metallic cavities containing adielectric material which were limited in Q-factor by the properties anddimensions of the metallic cavities. These prior art resonators werecommonly operated at cryogenic temperatures in order to obtain a betterQ-factor. However, to maintain cryogenic temperatures requires equipmentwhich is cumbersome and difficult to incorporate into a portable orcompact apparatus.

U.S. Pat. No. 5,712,605 to Flory and Taber describes a resonatorstructure that seeks to address these problems. The resonator describedin U.S. Pat. No. 5,712,605 is a complex stack of hollow cylinders andflat discs formed of dielectric material. The cylinders and discs areenclosed within a metal cavity, with the hollow cylinders and discsforming a series of axially aligned cavities. The length of thecylinders and the diameter of the discs determine the operating mode ofthe resonator. The resonator is described as offering a high Q-factor.

Although the resonator described in U.S. Pat. No. 5,712,605 offers ahigh Q-factor, there are several disadvantages associated with theresonator structure. These include the difficulty of manufacture and itssensitivity to vibration. The device is difficult to manufacture becausethe hollow cylinders must be perfectly coaxial or the operation of theresonator will be significantly impaired. Further, because the resonantcavities are defined by the dielectric discs and hollow cylinders, anyvibration or movement induced in one or more of the dielectric hollowcylinders or discs will result in a corresponding change in the shape ofthe resonant cavity, with a resulting change in the resonant frequency.This is referred to as mode breaking and has limited the usefulness ofthis resonator structure.

C. J. Maggiore et al describe a further resonator structure in theirpaper “Low-loss microwave cavity using layered-dielectric materials”,Appl. Phys. Lett. Vol 64 No 11, p1451. This resonator comprises ahollow, cylindrical copper cavity with one to four circular dielectricplates placed in parallel and axially spaced within the cavity. TheQ-factor of the resonator was observed by Maggiore et al to increase asmore dielectric plates were used.

Maggiore et al acknowledge at p1453 that although the Q-factor of theresonator at room temperature is high enough to have application tofrequency stabilised oscillators, it will be necessary to thermallystabilise the cavity. This is because the dielectric plates are held inthe cavity by means of circumferential grooves cut in the cavity wall.Copper has a thermal expansion coefficient of 16.8×10⁻⁶; thus a 1°Celsius temperature change will produce a 3.5 MHz change in operatingfrequency of a 19 GHz resonator, which is due to the change in spacingbetween the dielectric plates resulting from the expansion/contractionof the copper cavity.

DISCLOSURE OF THE INVENTION

Throughout the specification, unless the context requires otherwise, theword “comprise” or variations such as “comprises” or “comprising”, willbe understood to imply the inclusion of a stated integer or group ofintegers but not the exclusion of any other integer or group ofintegers.

In accordance with one aspect of this invention, there is provided amulti-layer microwave resonator, comprising:

-   -   a cavity having an inner surface formed from an electrically        conductive material;    -   a plurality of pieces of dielectric materials stacked on top of        each other to form a contiguous body, the body being provided in        the cavity;    -   wherein the dielectric materials of the pieces are chosen such        that the dielectric constant of the pieces alternate between a        relatively high dielectric constant and a relatively low        dielectric constant.

Preferably, the dielectric materials of the pieces are chosen such thatthe thermal coefficient of dielectric constant of the pieces alternatebetween a positive thermal coefficient of dielectric constant and anegative thermal coefficient of dielectric constant.

Preferably, the body includes a central piece of dielectric materialhaving a relatively low dielectric constant.

Preferably, the central piece has a length substantially commensuratewith an integer multiple of one-half wavelength of a desired operatingfrequency in the dielectric material.

In one arrangement of the invention:

-   -   the body is formed of three pieces of dielectric materials,        arranged as a central piece of a first dielectric material and        two end pieces of a second dielectric material, the central        piece being provided between the two end pieces;    -   the central piece of dielectric material having a length        substantially commensurate with an integer multiple of one half        wavelength of a desired operating frequency in said first        dielectric material;    -   each end piece having a length substantially commensurate with        an odd integer multiple of one-half wavelength of the desired        operating frequency in the second dielectric material;    -   the dielectric constant of the second dielectric material being        greater than the dielectric constant of the first dielectric        material.

A preferred form of this arrangement further comprises:

-   -   an even plurality of intermediate pieces of dielectric materials        provided between the central piece and each end piece;    -   each intermediate piece being formed from either the first        dielectric material or the second dielectric material;    -   each intermediate piece having a length substantially        commensurate with an odd integer multiple of one-quarter        wavelength of the desired operating frequency in whichever of        the first or second dielectric material the intermediate piece        of formed from;    -   the intermediate pieces provided between the central piece and        each end piece comprise an equal number of intermediate pieces        formed from the first dielectric material and intermediate        pieces formed from the second dielectric material;    -   the intermediate pieces being arranged such that the pieces of        dielectric materials forming the body alternate between pieces        formed from the second dielectric material and pieces formed        from the first dielectric material.

In an alternative arrangement of the invention:

-   -   the body is formed of five pieces of dielectric materials,        arranged as a central piece of a first dielectric material, two        intermediate pieces of a second dielectric material and two end        pieces of the first dielectric material, the central piece being        provided between the two intermediate pieces, the central piece        and the intermediate pieces being provided between the two end        pieces;    -   the central piece of dielectric material having a length        substantially commensurate with an integer multiple of one-half        wavelength of a desired operating frequency in said first        dielectric material;    -   each intermediate piece having a length substantially        commensurate with an odd integer multiple of one-quarter        wavelength of the desired operating frequency in the second        dielectric material;    -   each end piece having a length substantially commensurate with        an odd integer multiple of one-quarter wavelength of the desired        operating frequency in the first dielectric material;    -   the dielectric constant of the second dielectric material being        greater than the dielectric constant of the first dielectric        material.

A preferred form of this arrangement further comprises:

-   -   an odd plurality of intermediate pieces of dielectric materials        are provided between the central piece and each end piece;    -   each intermediate piece being formed from either the first        dielectric material or the second dielectric material;    -   each intermediate piece having a length substantially        commensurate with an odd integer multiple of one-quarter        wavelength of the desired operating frequency in whichever of        the first or second dielectric material said intermediate piece        is formed from;    -   the intermediate pieces provided between the central piece and        each end piece comprise alternate between an intermediate piece        formed from the second dielectric material and an intermediate        piece formed from the first dielectric material;    -   the intermediate pieces being arranged such that the pieces of        dielectric materials forming the body alternate between pieces        formed from the first dielectric material and pieces formed from        the second dielectric material.

Preferably, each intermediate piece formed from the second dielectricmaterial has an aperture formed centrally therein.

Preferably, each intermediate piece formed from the first dielectricmaterial has an aperture formed centrally therein.

Preferably, the central piece has an aperture formed centrally therein.

Preferably, each end piece formed has an aperture formed centrallytherein.

Preferably, the central piece has an opening formed therein forreceiving test substances.

Preferably, the first dielectric material is sapphire.

Preferably, the second dielectric material is rutile.

Preferably, the pieces of dielectric material are substantiallycylindrical.

Preferably, the cavity comprises a cylindrical wall and two ends, thebody being contained between the ends of the cavity.

In one arrangement, the cylindrical wall is spaced from the body.

In an alternative arrangement, the cylindrical wall abuts the body.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the followingdescription of six specific embodiments thereof and the accompanyingdrawings, which;

FIGS. 1(a) and 1(b) are elevation and plan cross-sections, respectively,of a multi-layer microwave resonator in accordance with a first aspectof this invention;

FIGS. 2(a) and 2(b) are elevation and plan cross-sections, respectively,of a multi-layer microwave resonator in accordance with a secondembodiment of this invention;

FIG. 2(c) is an elevation cross-section of the muti-player microwaveresonator shown in FIGS. 2(a) and 2(b) showing the distribution ofelectromagnetic fields within the microwave resonator;

FIGS. 3(a) and 3(b) are elevation and plan cross-sections of amulti-layer microwave resonator in accordance with a third embodiment ofthis invention; FIGS. 4(a) and 4(b) are elevation and plancross-sections of a multi-layer microwave resonator in accordance with afourth embodiment of the invention, in which FIG. 4(a) also includes aillustrative representation of electromagnetic fields within themicrowave resonator;

FIGS. 5(a) and 5(b) are elevation and plan cross-sections of amulti-layer microwave resonator in accordance with a fifth embodiment ofthis invention; and

FIGS. 6(a) and 6(b) are elevation and plan cross-sections of amulti-layer microwave resonator in accordance with a sixth embodiment ofthis invention.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

The embodiments are directed towards multi-layer microwave resonatorswhich can be used in a variety of applications. The microwave resonatorsare intended to provide a relatively high Q-factor and operate in a loworder mode to reduce spurious modes. The relatively solid constructionsreduces the vibrational sensitivity of the device.

The first embodiment is directed towards a microwave resonator 10comprising a cavity 12 and a body 14 that is formed from five pieces ofdielectric material 16(a)-16(e) stacked on top of each other, as shownin FIGS. 1 a and 1 b.

The cavity 12 comprises a cylindrical wall 18 and two end sections 20(a)and 20(b). The cylindrical wall 18 and the end sections 20(a) and 20(b)are formed from copper. In other embodiments, the wall and end sectionsmay be formed from other electrically conductive materials, or may havean inner surface coated with such a material. For best performance, itis preferred that the electrically conductive material have a lowimpedance, such as silver or copper.

The body 14 is provided within the cavity 12 coaxially with thecylindrical wall 18. The body 14 is held in place between the endsections 20(a) and 20(b). If desired, recesses of an appropriate shapemay be formed in the end sections 20(a) and 20(b) to more securely holdthe body 14 in position within the cavity 12.

The cylindrical wall 18 is spaced from the body 14 in the embodiment todefine an annular air filled or vacuum space 22.

The body 14 may be retained within the cavity 12 by holding it incompression between the end sections 20 a and 20 b of the cavity 12.

Further, the pieces of dielectric material 16 a-16 e may be heated sothat adjacent pieces 16 a-16 e fuse together to form a single body,though this is not essential.

Each of the pieces of dielectric material 16(a)-16(e) are solidcylinders in shape in this embodiment. The piece 16(c) forms a centralpiece of the body 14, having pieces 16(b) then 16(a) stacked on top ofit and pieces 16(d) and 16(e) stacked below it. The pieces 16(a) and16(e) form end pieces of the body 14, with the pieces 16(b) an 16(d)being intermediate pieces between the end pieces 16(a) and 16(e) and thecentral piece 16(c).

The central piece 16(c) and the end pieces 16(a) and 16(e) are formed ofsapphire as the dielectric material in this embodiment. The intermediatepieces 16(b) and 16(d) are formed with rutile as the dielectric materialin this embodiment. Rutile is known to have a higher dielectric constantthan sapphire and so, in relative terms, the dielectric constant in thebody goes as low, high, low, high, low. In this regard, what isimportant is that the dielectric constant of the dielectric materialfrom which the pieces 16(b) and 16(d) are made from is higher than thedielectric constant of the dielectric material from the layers 16(a),16(c) and 16(e) are made from, rather than their absolute values.

A further advantage of rutile as a dielectric material is that itstemperature coefficient of dielectric constant is positive, whereas mostother dielectric materials have a negative dielectric constant. Whenused in the body 14, the rutile in pieces 16(b) and 16(d) act to offsetthe temperature coefficient of dielectric constant of the sapphire inpieces 16(a), 16(c) and 16(e). This reduces the sensitivity of theresonator 10 to temperature variations.

The length in the axial direction of each of the pieces of dielectricmaterials 16(a)-16(e) is determined according to the wavelength of adesired operating frequency within the respective piece of dielectricmaterial. In this regard, the central piece 16(c) has an axial lengthcorresponding with one half wavelength at the desired frequency, theintermediate pieces 16(b) and 16(d) have an axial length correspondingwith one quarter wavelength of the desired frequency, and end pieces16(a) and 16(e) each have a length corresponding with one quarterwavelength at the desired frequency. Although the axial length of thecentral piece 16(c) can be any multiple of one half wavelength, and theaxial length of pieces 16(a), 16(b), 16(d) and 16(e) can be any oddmultiple of one quarter wavelength, it is preferred that a singlemultiple is used to minimise spurious modes. It also minimises the sizeof the device where space is at a premium.

The operating frequency of the microwave resonator 10 can be tuned afollows. Firstly, coarse tuning can be achieved by selecting the axiallength of each of the pieces of dielectric materials 16(a)-16(e) asdescribed above. However, the machining process that creates the pieces16(a)-16(e) is not accurate enough to achieve exact dimensions. Thus,medium frequency tuning can be achieved by adjusting the diameter of thecylindrical wall 18, such as by machining. Fine adjustment of theoperating frequency can be achieved by temperature regulation.

The second embodiment is shown in FIGS. 2(a) and 2(b). FIG. 2(b) is across-section through lines A—A in FIG. 2(a). The second embodiment isdirected towards a multi-layer microwave resonator 110 of the samegeneral form as the microwave resonator 10 described in the firstembodiment. Like reference numerals are used to denote like parts tothose shown in the first embodiment, with 100 added thereto.

The multi-layer microwave resonator 110 differs from the microwaveresonator 10 in the first embodiment in that the intermediate pieces116(b) and 116(d) of rutile each have a circular aperture 124 formedtherein. The aperture 124 can be left empty or filled with a very lowloss, low dielectric constant dielectric material. It should beappreciated that the length of the pieces of dielectric material 116a-116 e do not need to necessarily have lengths exactly corresponding toa multiple of a quarter or half wavelength, as appropriate. Rather,these values provide guides for construction of the resonators. In someinstances, it may be desirable to vary the length of some of the piecesof dielectric material to optimise desired characteristics of theresonator. For example, the resonator shown in FIGS. 2 a and 2 b wereentered into a finite element electromagnetic analysis tool, withparameters that the length of each piece of dielectric material may bevaried slightly, and the characteristic to optimise was chosen as theQ-factor of the resonator. After several iteratiors of analysis, thestructure with the highest Q-factor is that shown in FIG. 2 c. As can beseen, the length of the end pieces has increased to substantially thatof the centre piece.

FIG. 2(c) also shows the distribution of electromagnetic energy withinthe Q-factor optimised resonator 110.

The third embodiment is directed towards a multi-layermicrowave-resonator 210, as shown in FIGS. 3(a) and 3(b). Like referencenumerals they used to denote like parts in those in the firstembodiment, with 200 added thereto.

The microwave resonator 210 differs from the microwave resonator 10 inthe first embodiment in that the body 214 in this embodiment is formedfrom nine pieces 216 a-216 i of dielectric materials. In the currentembodiment, the piece 216 e forms the central piece of the body 214,with intermediate pieces 216 d, 216 c, 216 b and finally end piece 216 astacked on top of it and intermediate pieces 216 f, 216 g, 216 h andfinally end piece 216 i stacked below it.

The pieces 216 a, 216 c, 216 e, 216 g and 216 i are formed fromsapphire. The pieces 216 b, 216 d, 210 f and 216 h are formed rutile.Each of the pieces 216 a-216 d and 216 f-216 i have an axial lengthcommensurate with one quarter wavelength in the corresponding dielectricmaterial. Increasing the number of layers offers a higher Q-factor, butat the expense of increased complexity of manufacture. Conceptually,further pieces of dielectric material can be added to a body adinfinitum, but each subsequent piece offers diminishing returns.

Further, in the microwave resonator 210 of the current embodiment, thecylindrical wall 18 abuts the body 214.

The fourth embodiment is directed towards a microwave resonator 310,shown in FIGS. 4(a) and 4(b). Like reference numerals they are used todenote like parts to those used in the first embodiment, with 300 addedthereto.

The microwave resonator 310 in the current embodiment is of the samegeneral form as the microwave resonator 10 in the first embodiment, theonly difference being that the diameter of the pieces 316 a-316 e aregreater than the corresponding pieces 16 a-16 e in the first embodiment.Further, the wall 318 abuts the body 314 in this embodiment.

The lines marked B in FIG. 4(a) offer an illustrative representation ofthe electro-magnetic field present in the resonator 310.

The fifth embodiment is directed towards a microwave resonator 410,shown in FIGS. 5(a) and 5(b). Like reference numerals they are used todenote like parts to those used in the second embodiment, with 300 addedthereto.

The microwave resonator 410 in the current embodiment is of the samegeneral form as the microwave resonator 110 in the second embodiment,the only difference being that the central piece 416 c and the endpieces 416 a and 416 e each have an aperture 480 formed centrallytherein which extends through each piece. This arrangement may increasethe Q-factor of the resonator 410 compared with the resonator 110.

The sixth embodiment is directed towards a microwave resonator 510,shown in FIGS. 6(a) and 6(b). Like reference numerals they are used todenote like parts to those used in the first embodiment, with 500 addedthereto.

The microwave resonator 510 in the current embodiment is of the samegeneral form as the microwave resonator 10 in the first embodiment, theonly difference being that the body 514 is formed from seven pieces ofdielectric material 516 a-516 g. Thus, the end pieces 516 a and 516 gare formed from a dielectric material having a relatively highdielectric constant it should be appreciated that the scope of thisinvention is not limited to the particular embodiments described above.

For example, the multi-layer microwave resonator can be made with morethan 7 layers or less than 5, as desired. Further, the diameter of thepieces of dielectric material 16(a)-16(e) can be adjusted according torequirements.

Further, it is envisaged that an opening can be provided within the body14, preferably within the central piece 16(c) to receive test substancestherein in order to examine the effects of exposure to microwaveenergies.

Further, it is envisaged that dielectric materials other than sapphireand rutile can be used.

1. A multi-layer microwave resonator, comprising: a cavity having aninner surface formed from an electrically conductive material; aplurality of pieces of dielectric materials stacked on top of each otherto form a contiguous body, the body being provided in the cavity; thebody including a central piece of a first dielectric material having arelatively low dielectric constant, the central piece having a lengthsubstantially commensurate with an integer multiple of one-halfwavelength of a desired operating frequency in the dielectric material;wherein the dielectric materials of the pieces are chosen such that thedielectric constant of the pieces alternate between a relatively highdielectric constant and a relatively low dielectric constant.
 2. Aresonator as claimed in claim 1, wherein the dielectric materials of thepieces are chosen such that the thermal coefficient of dielectricconstant of the pieces alternate between a positive thermal coefficientof dielectric constant and a negative thermal coefficient of dielectricconstant.
 3. A resonator as claimed in claim 1, wherein: the body isformed of three pieces of dielectric materials, arranged as a centralpiece of a first dielectric material and two end pieces of a seconddielectric material, the central piece being provided between the twoend pieces; the central piece of dielectric material having a lengthsubstantially commensurate with an integer multiple of one-halfwavelength of a desired operating frequency in said first dielectricmaterial; each end piece having a length substantially commensurate withan odd integer multiple of one-half wavelength of the desired operatingfrequency in the second dielectric material; the dielectric constant ofthe second dielectric material being greater than the dielectricconstant of the first dielectric material.
 4. A resonator as claimed inclaim 1, wherein: the body is formed of five pieces of dielectricmaterials, arranged as a central piece of a first dielectric material,two intermediate pieces of a second dielectric material and two endpieces of the first dielectric material, the central piece beingprovided between the two intermediate pieces, the central piece and theintermediate pieces being provided between the two end pieces; thecentral piece of dielectric material having a length substantiallycommensurate with an integer multiple of one-half wavelength of adesired operating frequency in said first dielectric material; eachintermediate piece having a length substantially commensurate with anodd integer multiple of one-quarter wavelength of the desired operatingfrequency in the second dielectric material; each end piece having alength substantially commensurate with an odd integer multiple ofone-quarter wavelength of the desired operating frequency in the firstdielectric material; the dielectric constant of the second dielectricmaterial being greater than the dielectric constant of the firstdielectric material.
 5. A resonator as claimed in claim 4, wherein: anodd plurality of intermediate pieces of dielectric materials areprovided between the central piece and each end piece; each intermediatepiece being formed from either the first dielectric material or thesecond dielectric material; each intermediate piece having a lengthsubstantially commensurate with an odd integer multiple of one-quarterwavelength of the desired operating frequency in whichever of the firstor second dielectric material said intermediate piece if formed from;the intermediate pieces provided between the central piece and each endpiece comprise alternate between an intermediate piece formed from thesecond dielectric material and an intermediate piece formed from thefirst dielectric material; the intermediate pieces being arranged suchthat the pieces of dielectric materials forming the body alternatebetween pieces formed from the first dielectric material and piecesformed from the second dielectric material.
 6. A resonator as claimed inclaim 3, further comprising: an even plurality of intermediate pieces ofdielectric materials provided between the central piece and each endpiece; each intermediate piece being formed from either the firstdielectric material or the second dielectric material; each intermediatepiece having a length substantially commensurate with an odd integermultiple of one-quarter wavelength of the desired operating frequency inwhichever of the first or second dielectric material the intermediatepiece of formed from; the intermediate pieces provided between thecentral piece and each end piece comprise an equal number ofintermediate pieces formed from the first dielectric material andintermediate pieces formed from the second dielectric material; theintermediate pieces being arranged such that the pieces of dielectricmaterials forming the body alternate between pieces formed from thesecond dielectric material and pieces formed from the first dielectricmaterial.
 7. A resonator as claimed in any one on claims 6 to 5, whereineach intermediate piece formed from the second dielectric material hasan aperture formed centrally therein.
 8. A resonator as claimed in anyone of claims 6 to 5, wherein each intermediate piece formed from thefirst dielectric material has an aperture formed centrally therein.
 9. Aresonator as claimed in any one of claims 6 to 5, wherein the centralpiece has an aperture formed centrally therein.
 10. A resonator asclaimed in any one of claims 6 to 5, wherein each end piece formed hasan aperture formed centrally therein.
 11. A resonator as claimed in anyone of claims 3 to 5, wherein the central piece has an operating formedtherein for receiving test substance.
 12. A resonator as claimed inclaim 1, wherein the first dielectric material is sapphire.
 13. Aresonator as claimed in claim 1, wherein the second dielectric materialis rutile.
 14. A resonator as claimed in claim 1, wherein the pieces ofdielectric material are substantially cylindrical.
 15. A resonator asclaimed in claim 1, wherein the cavity comprises a cylindrical wall andtwo ends, the body being contained between the ends of the cavity.
 16. Aresonator as claimed in claim 15, wherein the cylindrical wall is spacedfrom the body.
 17. A resonator as claimed in claim 15, wherein thecylindrical wall abuts the body.